Printed Circuit Boards

ABSTRACT

A printed circuit board to which a localised solder connection is to be made, the surface of said printed circuit board having a continuous or non-continuous coating of a composition comprising a halo-hydrocarbon polymer at a thickness of from 1 nm to 10 μm.

The present invention relates to articles such as those comprisingprinted circuit boards coated with a halo-hydrocarbon polymer.

Printed circuit boards (PCBs) are used in the electronics industry tomechanically support and electrically connect electrical and electroniccomponents. A PCB comprises a board or other substrate made of aninsulating material on which conductive tracks, typically made ofcopper, lie. These conductive tracks function as wires betweenelectrical components that are later attached to the board by, forexample, soldering. A large proportion of PCBs are manufactured bydepositing or otherwise adhering a layer of copper to the substrateboard, and then removing unwanted copper by chemical etching to leavecopper tracks in the required configuration. At this stage the blankPCBs may often be stored for variable periods of time, potentially up toseveral months, prior to attachment of the electronic components to thePCB by a soldering method.

The conductive tracks on the printed circuit board may be made from anyconductive material. The preferred material for the tracks is copper.Copper is the preferred material for the conductive tracks mainly due toits high electrical conductivity, but unfortunately copper is readilyoxidised in air leading to a layer of copper oxide, or tarnish, on thesurface of the metal. This oxidation is particularly evident if a longperiod of time has elapsed between manufacture of the blank PCB andattachment of the electrical components. The components are attached bysoldering, but the presence of an oxide layer on the copper tracks mayreduce the effectiveness of soldering. In particular dry joints, whichhave a tendency to fail during operation of the device, and weak jointswith low mechanical strength may be formed. Occasionally the joint willfail to make electrical contact altogether. Similar problems arise whenthe conductive tracks comprise conductive materials other than copper.

To minimise these problems, PCB manufacturers apply a range of coatings,or surface finishes, to the areas where soldering will be required.Metals such as tin, silver or a nickel/gold combination are frequentlyused. The processes for applying these finishes are all time consuming,requiring additional metals to be used, with consequent environmentalissues. There are potential health issues associated with some of theprocesses and materials. Further, some of the metals used, such as gold,are expensive. A similar approach involves coating the tracks with acoating comprising organic compounds such as benzimidazoles andparticles of solder-wettable metals or solder (see for example WO97/39610), thus preventing exposure of the tracks to oxidativeconditions. During soldering the organic layer is simply removed. Theseorganic coatings typically do not survive multiple heat cycles and havea relatively short storage life before processing.

It is apparent that the techniques adopted by manufacturers up until noware either expensive or time consuming (involving extra steps in themanufacturing process), or both, and deplete non-renewable resourcesincluding precious metals. There is a need for a cheaper and/or higherperformance method of preventing oxidation of the conductive tracksprior to attachment of electrical components by soldering.

A separate issue is that PCBs are often required in devices that areused in very harsh and corrosive environments. Under such conditions,the conductive tracks on the PCB may be corroded leading to a farshorter lifetime of the circuit board than would normally be expected.Such conditions may arise, for example, when a device is used in veryhumid environments, especially where microscopic droplets of watercontaining dissolved gases such as sulphur dioxide, hydrogen sulphide,nitrogen dioxide, hydrogen chloride, chlorine and water vapour form acorrosive solution. Additionally, droplets of moisture may form a thinfilm or corrosion deposits between conductive tracks on the PCB that maypotentially cause short circuits. In circumstances where PCBmanufacturers envisage the devices being used in hostile conditions,they have tended to coat the assembled PCB with a conformal coating of apolymer that forms a barrier to the environment. However such coatingsare expensive to apply and require an extra step in the manufacturingprocess to apply the coating after the PCB has been assembled, andgenerally an extra step later to remove it. This may also cause problemswhen reworking a damaged or failed PCB, or during testing to ascertainits performance and troubleshoot a problem. A cheaper and/or highperformance method of environmentally protecting the completed PCB wouldbe of great interest to manufacturers.

Another problem that may arise after soldering electronic components toa PCB is the formation of dendrites of metal compounds on the solderjoint. These dendrites can cause failure of the assembled PCB due toshort circuits between contacts. Dendrites are very fine metallicgrowths along a surface, resulting from electromigration, which formfern-like patterns. The growth mechanism for dendrites is wellunderstood, unlike “tin whiskers”, and requires the presence of moisturethat generates metal ions that are then redistributed byelectromigration in the presence of an electromagnetic field. Thecoating of the invention protects against the formation of dendrites bypreventing moisture reaching the surface of the PCB, which is wheredendrites normally grow. The coating provides additional protection asdendrite materials have low adhesion to the surface coating reducing theformation of dendrites between contacts and components.

The present invention provides a printed circuit board to which agenerally localised solder connection is to be made, the surface of saidprinted circuit board having a coating of a composition comprising oneor more halo-hydrocarbon polymers, in single or multiple layers, at athickness of from that of a monolayer (usually a few angstroms (A)) to10 μm, wherein there is no solder, or essentially no solder, betweensaid coating composition and the conductive tracks of said printedcircuit board. By polymer we include polymers formed in-situ from singleor multiple monomers, linear, branched, grafted and crosslinkedcopolymers, oligomers, multipolymers, multimonomer polymers, polymermixtures, grafted copolymers, blends and alloys of polymers, as well asinterpenetrating networks of polymers (IPNs).

The thickness of the coating is typically from 1 nm to 2 μm, moretypically from 1 nm to 500 nm, still more typically from 3 nm to 500 nm,still more typically from 10 nm to 500 nm, and most typically from 10 nmto 250 nm. The coating is preferably at a thickness of from 10 nm to 100nm, in various gradients, with 100 nm being a preferred thickness. Inanother embodiment, the thickness of the coating is 10 nm to 30 nm.However, the optimal thickness of the coating will depend on theproperties that are required of the PCB. For example, if very highenvironmental toughness is required (high corrosion and abrasionresistance), a thicker coating may be used. Additionally the coatingthickness may be optimised with different thicknesses at differentlocations across the PCB dependent on which feature is being optimised(for example, environmental protection versus Z axis conductivity). Thecoating thickness and flux composition can be varied to optimiseenvironmental protection characteristics and give particularly strongsolder joints.

The halo-hydrocarbon coating may be continuous, substantially continuous(particularly over surfaces to be soldered and non-soldering surfacesbetween or adjacent to them, and more particularly over substantiallyall exposed and vulnerable surfaces of the PCB), or non-continuous. Fora very high level of environmental protection, a substantiallycontinuous coating may be required. However, a non-continuous coatingmay be sufficient for other purposes.

By halo-hydrocarbon polymer it is meant a polymer with a straight orbranched chain or ring carbon structure with 0, 1, 2 or 3 halogen atomsbound to each carbon atom in the structure. The halogen atoms could bethe same halogens (for example fluorine) or a mixture of halogens (forexample fluorine and chlorine). The term “halo-hydrocarbon polymer” asused herein includes polymers that contain one or more unsaturatedgroups, such as carbon-carbon double and triple bonds, and polymer thatcontain one or more heteroatoms (atoms which are not C, H or halogen),for example N, S or O. Currently we prefer, however, that the polymercontains substantially no unsaturation (because unsaturation oftenresults in reduced stability) and substantially no such heteroatoms.Preferably the polymer contains less than 5% heteroatoms as a proportionof the total number of atoms in the polymer. Preferably the polymercontains less than 5% carbon-carbon double or triple bonds as aproportion of the total number of carbon-carbon bonds. The molecularweight of the polymer is preferably greater than 1000 amu.

The polymer chains may be straight or branched, and there may becrosslinking between polymer chains. The halogen may be fluorine,chlorine, bromine or iodine. Preferably the polymer is afluoro-hydrocarbon polymer, a chloro-hydrocarbon polymer or afluoro-chloro-hydrocarbon polymer wherein 0, 1, 2 or 3 fluoro or chloroatoms are bonded to each carbon atom in the chain.

Examples of preferred polymers include:

-   PTFE, PTFE type material, fluorinated-hydrocarbons,    chlorinated-fluorinated-hydrocarbons, halogenated-hydrocarbons,    halo-hydrocarbons or co-polymers, oligomers, multipolymers,    multimonomer polymers, polymer mixtures, blends, alloys, branched    chain, grafted copolymers, cross-linked variants of these materials    and also interpenetrating polymer networks (IPNs).-   PCTFE (polychlorotrifluoroethylene) and copolymers, oligomers,    multipolymers, multimonomer polymers, polymer mixtures, blends,    alloys, branched chain, grafted copolymers, cross-linked variants of    this material and also interpenetrating polymer networks (IPNs).-   EPCTFE (ethylene copolymer of polychlorotrifluoroethylene) and    copolymers, oligomers, multipolymers, multimonomer polymers, polymer    mixtures, blends, alloys, branched chain, grafted copolymers,    cross-linked variants of this material and also interpenetrating    polymer networks (IPNs).-   Other fluoroplastics including the materials below and co-polymers,    oligomers, multipolymers, multimonomer polymers, polymer mixtures,    blends, alloys, branched chain, grafted copolymers, cross-linked    variants of these materials as well as interpenetrating polymer    networks (IPNs): ETFE (copolymer of ethylene and    tetrafluoroethylene), FEP (copolymer of tetrafluoroethylene and    hexafluoropropylene), PFA (copolymer of tetrafluoroethylene and    perfluorovinyl ether), PVDF (polymer of vinylidenefluoride), THV    (copolymer of tetrafluoroethylene, hexafluoropropylene and    vinylidenefluoride), PVDFHFP (copolymer of vinylidene fluoride and    hexafluoropropylene), MFA (copolymer of tetrafluoroethylene and    perfluoromethylvinylether), EFEP (copolymer of ethylene,    tetrafluoroethylene and hexafluoropropylene), HTE (copolymer of    hexfluoropropylene, tetrafluoroethylene and ethylene) or copolymer    of vinylidene fluoride and chlorotrifluoroethylene and other    fluoroplastics.

Most preferably the polymer is a polytetrafluoroethylene (PTFE) typematerial, and in particular polytetrafluoroethylene (PTFE).

A lower wettability may be achieved by using a coating in which thehalo-hydrocarbon is a highly branched polymer a copolymer, polymer blendor a polymer mixture.

It is desirable that the coating composition have any one or more, andpreferably substantially all, of the following properties: capable ofbeing deposited as continuous films, free of cracks, holes or defects;relatively low gaseous permeability which provides a significant barrierto gaseous permeation and avoids gaseous corrosion and oxidation‘through’ the coating; the ability to selectively solder through withoutthe need for prior removal and to achieve good solder joints comparableto other currently available surface finishes; the ability to withstandmultiple heat cycles; chemical resistance to corrosive gases, liquidsand salt solutions, particularly environmental pollutants; exhibit lowsurface energy and ‘wettability’; to be stable inert material at normalPCB temperatures; have good mechanical properties, including goodadhesion to PCB materials and good mechanical abrasion resistance;improved electrostatic protection; relatively low liquid and saltsolution permeability, to avoid liquid corrosion ‘through’ the coating;and generally be environmentally beneficial compared to existingprocesses when used in this application.

The invention can also provide other electrical and/or electronicdevices, or other articles (such as pipes or other plumbing apparatus)to which solder connections are to be made, having such a coating. Forexample, the invention could be used to coat the bare wires (especiallycopper wires) used in wire bonding techniques. Wire bonding is a methodof making interconnections between an integrated circuit in bare dieform and the leadframe inside the integrated circuit or between the baredie and a PCB. The wire used has traditionally been gold or aluminum butmore recently there has been a considerable interest in using copperwire for a number of reasons including the significant cost differentialwith gold. In wire bonding, two jointing methods are commonly used,wedge bonding and ball bonding, both of which use different combinationsof heat, pressure, and ultrasonic energy to make a weld at either orboth ends of the wire. To achieve a good bond both the wire and the padto which it is bonded must be free of contaminants including oxidation.It is standard practice to apply a gold finish to the bond pad toprevent oxidation. The coating of the present invention on a copper bondpad will also provide an oxidation free surface, allowing wire bondingjoints to be made using gold, aluminum or copper wire, either by wedgebonding or ball bonding but at a significantly lower cost than thestandard gold finish on the bond pad. Where copper wire is being used itis also beneficial to apply the halo-hydrocarbon coating to the wire toprevent oxidation after the wire has been made and prior to storage.Also, the halo-hydrocarbon coating provides additional oxidationprotection during the bonding process. Alternatively, in anotherembodiment of the invention, the electrodes of electronic componentscould be coated. The polymer coating preferably provides a good barrierto the permeation of atmospheric gases and liquids, most importantlyoxygen, which would normally react with the conductive tracks, typicallycopper tracks, to form a layer of tarnish, typically copper oxide, onthe surface of the track. As a result, the coated circuit board may bestored for long periods of time, up to several months or years, withoutdamaging oxidation of the conductive tracks occurring. Opticalmicroscopy, scanning electron microscopy and back scattering electronimaging have been used to investigate the nature, continuity andthickness of the coating. Energy dispersive analysis by X-rays has beenused to map the levels and distribution of halogens in the coating.Measurements of the surface activation and surface wettability usingchemical solvent solutions provide an indication of the potential to actas a protective coating.

Once the manufacturer is ready to install components on the blank PCB,there is no requirement to clean the PCB or to remove the coating layerprior to the soldering process. This arises because, surprisingly, thehalo-hydrocarbon polymer used provides a coating that has the unusualproperty that it may be soldered through to form a solder joint betweenthe conductive track on the board and the electrical component. Flux isgenerally required in this soldering technique. In the extreme, asoldering process using heat alone could be used to selectively “remove”the coating, for example laser soldering. Welding, laser-enhancedwelding, ultrasonic welding or use of conductive adhesives are furtheralternatives. Another possible technique is wave soldering; thistechnique may require selective fluxing. The solder used may be leadedsolder or lead-free solder. There is generally no reduction in thestrength of the solder joint as might be expected, and indeed undercertain circumstances the solder joint may be stronger than a standardsolder joint. Furthermore, under certain circumstances, the presentinvention may prevent dendrite formation on the solder bonds,particularly when lead-free solder is used.

Thus, the present invention provides an alternative technique toapplying surface coatings of metals (such as tin, silver, nickel andgold) to the conductive tracks of PCBs to prevent oxidation of theconductive tracks prior to soldering. The present invention has theadvantage that it is based on a low cost process, it does not use toxicmetals such as nickel, it is environmentally friendly and it is saferthan current industrial metal plating processes. It also simplifies thePCB manufacturing process and is compatible with current industrialsoldering processes. In addition it has the extra benefit of “solderthrough” properties, whereby the need to remove the coating beforesoldering is avoided.

A further feature of the halo-hydrocarbon polymer coating is that it isonly removed in the areas where solder and/or flux is applied. Thus, inthe areas of the PCB where components are not attached by selectivesoldering the coating remains intact, maintaining a protective layerover the board and conductive tracks, which provides a barrier tocorrosion by atmospheric gases such as sulphur dioxide, hydrogensulphide, nitrogen dioxide, hydrogen chloride, chlorine and water vapourand other corrosive materials, thus avoiding corrosion by environmentalpollutants. The halo-hydrocarbon polymer coating is also substantiallyimpermeable to liquids and corrosive liquids. Consequently it ispossible to attach components to the circuit board in a series of stepswith significant periods of time elapsing between each step; this couldprovide a number of advantages to the manufacturer. This coating is notdestroyed by the soldering process other than in the selected solderareas, therefore in the non-soldered areas the PCB can be reprocessedand/or reworked by soldering at a later stage. Furthermore, onceassembly of the PCB is completed, the unsoldered areas of the PCB remaincoated with the halo-hydrocarbon polymer that forms a permanent barrierto environmental corrosion. There is no need for further costlyover-coating steps such as conformal coating.

The conductive tracks on the printed circuit board may comprise anyconductive material. Possible materials from which the conductive tracksmay be made are metals such as copper, silver, aluminium or tin, orconductive polymers or conductive inks. The preferred material for thetracks is copper. Conductive polymers tend to absorb water and swell,and thus coating conductive polymers with a halo-hydrocarbon polymerlayer can prevent water absorption.

Another feature of the coated PCB of the invention is that the z-axisimpedance is very low compared to the impedance in the x- and y-axis. Byz-axis it is meant the axis pointing into the plane of the PCB. Thecoating exhibits high impedance in the x- and y-axis, thus demonstratinggood insulating properties. However, the impedance is relatively low inthe z axis. This enables electrical contact to be made through thecoating without having to remove it. This is particularly advantageousfor applications such as keypads, switch contacts, test points and thelike. This characteristic can be further optimised by controlling theproperties of the coating e.g. by controlling the thickness of thelayer, its composition and the process conditions in the coating processand the nature of the coating process.

In summary the invention prevents oxidation of, and/or otherenvironmental damage, e.g. modulation of thermal stability, scratch,corrosion and chemical resistance and high barrier effect to theconductive tracks of the blank PCB and provides environmental protectionof the assembled PCB usually with one initial step of coating the blankPCB with a halo-hydrocarbon polymer.

The invention also provides a method of protecting a printed circuitboard which comprises providing a blank printed circuit board having anenvironmentally-exposed surface, and which has no solder, or essentiallyno solder, on said environmentally exposed surface, and applying to thatsurface to a thickness of a monolayer (usually a few angstroms (A)) to10 μm of a composition comprising a halo-hydrocarbon polymer by a thinfilm deposition technique such as plasma deposition, chemical vapourdeposition (CVD), molecular beam epitaxy (MBE), creation ofinter-penetrating polymer networks (IPNs), surface absorption ofmonolayers (SAMs) of polymers of monomers to form in-situ polymers,polymer alloys, or sputtering. Plasma enhanced-chemical vapourdeposition (PE-CVD), high pressure/atmospheric plasma deposition,metallo-organic-chemical vapour deposition (MO-CVD) and laserenhanced-chemical vapour deposition (LE-CVD) are alternative depositiontechniques. Liquid coating techniques such as liquid dipping, spraycoating, spin coating and sol/gel techniques are further alternatives.

The preferred method may depend on the thickness of coating that isrequired. Liquid coating techniques may be preferred for thickercoatings, while plasma deposition techniques may be preferred forthinner coatings. The thickness of the coating is typically from 1 nm to2 μm, more typically from 1 nm to 500 nm, still more typically from 3 nmto 500 nm, still more typically from 10 nm to 500 nm, and most typicallyfrom 10 nm to 250 nm. The coating is preferably at a thickness of from10 nm to 100 nm, with 100 nm being a preferred thickness. Thehalo-hydrocarbon polymer is preferably a fluoro-hydrocarbon polymer, achloro-hydrocarbon polymer or a fluoro-chloro-hydrocarbon polymer, whichmay also contain micro-pigments and small quantities of otherperformance additives (being a common practice in the polymer industry)and may for example be polytetrafluoroethylene (PTFE) type materials.The preferred method of applying the halo-hydrocarbon polymer to theblank PCB is plasma deposition, although all the other techniquesmentioned above would also be applicable.

Plasma deposition techniques are extensively used for deposition ofcoatings in a wide range of industrial applications. The method is aneffective way of depositing continuous thin film coatings using a dryand environmentally friendly technique. The PCBs are coated in a vacuumchamber that generates a gas plasma comprising ionised gaseous ions,electrons, atoms and neutral species. In this method, the PCB isintroduced into the vacuum chamber that is first pumped down topressures typically in the range 10⁻³ to 10 mbar. A gas is thenintroduced into the vacuum chamber to generate a stable gas plasma andone or more precursor compounds are then introduced into the plasma aseither a gas or liquid to enable the deposition process.

The precursor compounds are typically halogen-containing hydrocarbonmaterials, which are selected to provide the desired coating properties.When introduced into the gas plasma the precursor compounds are alsoionized/decomposed to generate a range of active species that will reactat the surface of the PCB, typically by a polymerisation process, togenerate a thin halo-hydrocarbon coating. Preferred precursor compoundsare perfluoroalkanes, perfluoroalkenes, perfluoroalkynes, fluoroalkanes,fluoroalkenes, fluoroalkynes, fluorochloroalkanes, fluorochloroalkenes,fluorochloroalkynes, or any other fluorinated and/or chlorinated organicmaterial (such as fluorohydrocarbons, fluorocarbons,chlorofluorohydrocarbons and chlorofluorocarbons).

In another aspect of the invention, the coating on the conductive trackof the PCB may comprise a very thin layer (for example 5 nm or less) ofmetal halide (preferably a metal fluoride, such as copper fluoride)directly in contact with the metal surface. In one embodiment, the metalhalide layer may be a monolayer or substantially a monolayer, or a fewmonolayers, or comprise a metal halide zone of layers at the surface.Such a metal halide layer may be very robust and inert, and preventsformation of oxide layers or other tarnishes which prevent effectivesoldering. The metal halide layer may form when active species in thegas plasma react with the metal surface or may be enhanced using ahigher concentration of fluorine species. The halo-hydrocarbon layer maythen be deposited in combination with the metal halide layer. The twolayers may be discrete, axially or spatially, or alternatively there maybe a graded transition from metal halide to halo-hydrocarbon. It ispossible that the metal halide layer protects the metal from oxidation,whilst the halo-hydrocarbon layer provides environmental protection fromcorrosive gases and/or liquids as well as oxidation protection.Furthermore, should the coating eventually be worn away by mechanicalabrasion, the underlying metal fluoride layer will prevent oxidationbuild up, enabling contact to still be made. The nature and compositionof the plasma deposited coating depends on a number of conditions: theplasma gas selected; the precursor compound used; the plasma pressure;the coating time; the plasma power; the chamber electrode arrangement;the preparation of the incoming PCB; and the size and geometry of thechamber. Typically the plasma deposition technique can be used todeposit thin films from a monolayer (usually a few angstroms (A)) to 10microns (preferably to 5 microns), depending on the above settings andconditions. The plasma technique itself only impacts the uppermostsurface of the PCB and is typically fully compatible with the PCBitself, causing no damage or other unwanted effects. An advantage ofplasma coating techniques is that the coating deposited accesses allsurfaces of the PCB, and thus vertical surfaces such as those onlyaccessible through holes in the PCB and any overhangs will also becovered. If a particular area of the PCB should not be coated withpolymer, for example gold contacts at the edge of the PCB, then theseareas can be masked during the plasma deposition process.

In a variant of the plasma process, it is possible to use the plasmamethod for in situ cleaning of the surface of the PCB prior to plasmadeposition using an active gas plasma. In this variant, an active gasplasma is used typically in the same chamber for PCB cleaning ahead ofintroduction of the precursor compound for the plasma deposition stage.The active gas plasma is based on a stable gas, such as hydrogen,oxygen, nitrogen, argon, methane, ethane, other hydrocarbons,tetrafluoromethane (CF₄), hexafluoroethane (C₂F₆), tetrachloromethane(CCl₄), other fluorinated or chlorinated hydrocarbons, other rare gases,or a mixture thereof. In one particular embodiment, the PCB could becleaned by the same material as to be deposited. For example, afluorinated or chlorinated hydrocarbon such as tetrafluoromethane (CF₄)or hexafluoroethane (C₂F₆) or hexafluoropropylene (C₃F₆) oroctafluoropropane (C₃F₈) could be used in the plasma method both toclean the surface of the PCB and lay down a layer of a halo-hydrocarbonpolymer and/or a layer of metal fluoride (or chloride).

The invention also provides a method of making a connection to a printedcircuit board coated with a composition that comprises ahalo-hydrocarbon polymer, which method comprises applying solder andflux to the printed circuit board at a temperature and for a time suchthat the solder bonds to the metal and the composition is locallydispersed and/or absorbed and/or vaporised and/or dissolved and/orreacted. The action of flux and increased temperature alone willgenerally interact with the halo-hydrocarbon polymer to remove thecoating locally from the area of the PCB to which the flux is applied.The temperature is typically 200° C. to 300° C., preferably 240° C. to280° C., and most preferably 260° C. In one embodiment, thehalo-hydrocarbon polymer may be dissolved and/or absorbed by the flux.We have found that there is often a balance between the temperaturerequired and the acidity or other aggressiveness of the flux. Thus,milder fluxes may suffice if higher temperatures are used, and viceversa. In another embodiment, we can take advantage of the self fluxingaction of copper fluoride at the copper surface and any decomposition ofthe polymer coating to release fluorine and/or HF to initiate fluxing(self fluxing). In the extreme, we have found that in certain cases theinvention may dispense with a flux if a sufficiently high temperature isused and so localised heating could be applied. Surprisingly, thecomposition is generally only removed specifically from the area wheresolder and/or flux is applied, and therefore the composition remainsattached to the surface of the PCB right up until the solder joint. Thisprovides advantageous environmental protection of the conductive tracksof the PCB right up to the solder joint.

The flux used in the invention could be a resin/rosin flux, an organicflux, an inorganic flux, a halide free flux, a no-clean flux, ano-residue flux or a low solids flux. A resin/rosin flux could forexample be a synthetic resin or a natural rosin. An organic flux couldfor example be: an organic acid such as lactic acid or an acrylic acids;an organic salt such as dimethylammonium chloride (DMA HCl); or anorganic amine such as urea. An inorganic flux could for example be: aninorganic salt such as zinc chloride, sodium chloride, potassiumchloride or sodium fluoride; or an inorganic acid such as hydrochloricacid or nitric acid. An example of a no-clean flux is a rosin flux.Other fluxes used more widely for industrial applications such asgeneral soldering, brazing and welding, or to clean or etch a metalsurface (for example borax) could also be used in the present invention.The flux used in this method is typically a mild flux such as a“no-clean” flux that does not require a subsequent step of cleaning thePCB. The flux may optionally be part of a soldering paste. The choice offlux may depend on the nature of the coating, particularly the thicknessand composition of the coating. A thicker more resistive coating mayrequire use of a more aggressive flux. A composition comprising theactive ingredient or ingredients of flux that remove thehalo-hydrocarbon composition from the board could also be used in thepresent invention in place of flux.

Further, the invention provides a use of a composition comprising ahalo-hydrocarbon polymer for environmentally-protecting a printedcircuit board to which a solder connection is to be made through thecomposition, without its prior removal, by dispersal and/or absorptionand/or vaporisation of the composition optionally in the presence of aflux.

The environment may contain gaseous agents such as sulphur dioxide,hydrogen sulphide, nitrogen dioxide, hydrogen chloride, chlorine, ozoneor water vapour, or liquids such as water, water in which the corrosivegases above are dissolved, salt solutions or other spillages. Such gasesare commonly present in highly polluted environments such as cities withatmospheric pollution problems. One particular environmental hazard thatthe present invention protects PCBs against is atmospheric moisture inwhich one or more of the corrosive gases listed above is dissolved. Wehave found that the invention is able to protect PCBs against such harshenvironments.

The invention also provides the use of a composition comprising ahalo-hydrocarbon polymer for providing long-term storage stability for ablank printed circuit board to which a solder connection is to be made.As mentioned above, the conductive tracks on PCBs tend to oxidise ifleft exposed to the atmosphere. The oxidation reactions are normally theformation of metal oxides by reaction with atmospheric oxidation, butalso include other oxidation reactions, for example where copper metalis oxidised to for example Cu⁺ or Cu²⁺. The composition of the inventionprevents these oxidation reactions so that a blank PCB can be stored forlong periods of time, without oxidation of the conductive tracksoccurring. Thus, after a long period of storage, good solder connectionscan be made to the PCB by standard soldering techniques, preferably inthe presence of flux, without any pre-cleaning steps.

The invention also provides the use of a composition comprising ahalo-hydrocarbon polymer to prevent oxidation and/or corrosion of theconductive tracks of a blank printed circuit board prior to theapplication of solder to said conductive tracks and/or the formation ofa solder connection.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a soldering profile of a reflow oven used with a commercialsolder paste containing lead.

FIG. 2 shows a soldering profile of a reflow oven used with a lead-freecommercial solder paste.

FIG. 3 is an image of a coated PCB of the invention with a water dropleton the surface, demonstrating the low surface energy, low wettability,liquid impermeable nature of the surface coating.

FIG. 4 is a cross-section image of strong solder joint made by solderingthrough the coating on a PCB of the invention.

FIG. 5 is a cross-section image of strong solder joint formed on a PCBof the invention, demonstrating the formation of good quality copper-tinintermetallics on the upper side of lower copper surface.

FIG. 6 is an SEM (Scanning Electron Microscopy) image of the edge of a 1μtm thick coating polymer on a PCB of the invention, shown at ×30,000magnification.

FIG. 7 is a BEI Image (Back Scattering Electron Image) showing anexample area of coated PCB of the invention, demonstrating coatingcontinuity in excess of 99.8% coverage.

FIG. 8 is a SEM/EDX image showing a region of coating removedselectively from a PCB of the invention by the action of flux at atemperature, for a nominal 1 micron thick coating. The image on the leftshows where flux has been selectively applied. The image on the rightshows that the coating has selectively been removed in the area to whichflux was applied.

FIG. 9 is an EDX spectrum showing the carbon/fluorine composition of thecoating on the copper of a PCB of the invention.

FIG. 10 is an image of IC component legs ripped from a soldered PCB ofthe invention, demonstrating strong solder joints. Under severe testingthe joints finally fail by fracture of the copper pad to board substratebond, rather than at the solder joint.

FIG. 11 is an image of soldered pads ripped from soldered PCB of theinvention, demonstrating strong solder joints. Under severe testing thejoints finally fail by fracture of the copper pad to board substratebond, rather than at the solder joint.

FIG. 12 is an SEM image and an EDX image showing the presence of polymercoating right up to a solder joint edge formed on a PCB of theinvention.

FIG. 13 is an optical microscopy image showing a series of good qualitysolder joints formed on a PCB of the invention.

FIG. 14 a is an XPS spectrum of a set of thin coatings of the inventionshowing various contributions from C-F and Cu-F materials.

FIG. 14 b is an XPS spectrum showing C-F containing material for a thickcoating.

EXAMPLES Preparation of Coated Printed Circuits Boards

Printed circuit boards that had been etched and cleaned but had not hadthe surface finish applied were obtained from a manufacturer. Theseboards were then treated by plasma deposition to generate thehalogen-containing coating. The PCB was introduced into the vacuumchamber that was first pumped down to pressures in the range 10⁻³ to 10mbar. A gas was then introduced into the vacuum chamber to generate astable gas plasma and a halogen-containing precursor hydrocarboncompound was then introduced into the plasma to enable the depositionprocess. When introduced into the gas plasma the precursor compound alsodecomposed/ionised to generate a range of active species that reacted atthe surface of the PCB to generate a thin halogen-containing coating. Anumber of experiments were carried out on these treated boards.

Example 1

A commercial solder paste containing lead was applied by hand dispensingfrom a syringe onto a number of the component pads on one side of thePCB. Several integrated circuits were placed onto the pads that hadsolder paste on them. The PCB was then put into a reflow oven where thesoldering profile had been set up as shown in FIG. 1. Subsequently, thejoints were examined visually using a microscope, where they were foundto have good wetting characteristics. Some of the joints were thenpulled apart by prising the component up with a tool. In each case theleg of the integrated circuit pulled out of the solder, leaving thejoint to the PCB pad intact.

Example 2

The above tests were repeated using lead-free solder paste with amodified reflow profile as shown in FIG. 2, with similar results.

Example 3

Flux only was applied to regions of two PCB's and they were heated up to260 C for ten seconds and five minutes. Examination showed that thecoating was no longer present in the areas where flux had been appliedon either of the PCBs. The coating however remained intact in the areaswhere flux had not been applied.

Example 4 Shear Strength Test

Eight assemblies with four PCB finishes were prepared for shear testing.

There were two assemblies for each PCB finish. Each assembly had seven1206 chip resistors and four 0805 chip capacitors assembled. Fourteen1206 resistors and eight 0805 capacitors from each assembly finish wereshear tested to determine the Ultimate Shear Strength (USS) of thesolder joints for each finish assembly.

Test Conditions

The board was mounted in a shear tester. The stand-off height of thechisel tool above the PCB surface was 80 m, and the width of chisel toolis 2 mm. During each test, the shear tool was moved forward at a definedspeed of 100 μm/s against the test component, and the force wasmonitored until the solder joint attachment broke. The shear tester usedis the Dage Series 4000, with a DS 100 testing head.

Results of Initial Shear Strength Tests

OSP ENIG PTFE 72.03 69.47 73.14 71.29 72.88 63.20 68.72 68.35 75.4968.10 70.21 77.28 67.70 67.89 77.08 79.03 68.59 69.99 73.21 74.22 67.4674.35 72.32 72.31 79.57 68.95 70.50 66.34 66.05 65.95 78.15 82.6  61.8070.62 79.43 61.52 72.07 76.98 71.09 68.16 70.31 71.15 72.10 ± 4.34 72.01± 4.78 69.85 ± 5.25

Example 5

The table of PCB surface energies below shows increased hydrophobicitywith coating process time:

Coating Process Time (min) 0 1 5 7.5 10 15 20 30 50 Surface 50 46 <26<26 <26 <26 <26 <26 <26 Energy (mN/m) Note The Limit of Detection of theSurface Energy Measurement Method ca. 26 mN/m.

1-12. (canceled)
 13. A method, comprising: attaching a plurality ofconductive tracks to at least one surface of a substrate comprising aninsulating material; depositing a coating on the at least one surface ofthe substrate, the coating covering at least a portion of the pluralityof conductive tracks, the coating comprising at least onehalo-hydrocarbon polymer; and after depositing the coating, solderingthrough the coating to form a solder joint between an electricalcomponent and at least one conductive track attached to the substrate,the solder joint abutting the coating.
 14. The method of claim 13,wherein the coating has a thickness from 1 nanometer to 10 micrometers.15. The method of claim 13, wherein the coating has a thickness from 10nanometers to 100 nanometers.
 16. The method of claim 13, wherein thecoating is deposited directly on the plurality of conductive tracks suchthat there is essentially no solder between the coating and theplurality of conductive tracks.
 17. The method of claim 13, wherein theat least one halo-hydrocarbon polymer comprises one or morefluoro-hydrocarbons.
 18. The method of claim 13, wherein less than fivepercent of the total number of atoms in the at least onehalo-hydrocarbon polymer are heteroatoms.
 19. The method of claim 18,wherein: the at least one halo-hydrocarbon polymer has a straight orbranched chain structure; the at least one halo-hydrocarbon polymercomprises at least one heteroatom; and the at least one heteroatomcomprises at least one of: nitrogen; sulfur; and oxygen.
 20. The methodof claim 13, wherein depositing the coating comprises: forming a firstlayer directly on the plurality of conductive tracks, the first layercomprising a metal halide; and forming a second layer on the firstlayer, the second layer comprising at least one halo-hydrocarbonpolymer.
 21. The method of claim 20, wherein the first layer comprisinga metal halide allows self fluxing during soldering of the at least oneelectrical component.
 22. The method of claim 13, wherein depositing thecoating comprises: forming a first layer directly on the plurality ofconductive tracks, the first layer comprising a metal halide thatprotects the plurality of conductive tracks from oxidation; and forminga second layer on the first layer, the second layer comprising one ormore halo-hydrocarbon polymers that protect the plurality of conductivetracks from corrosion.
 23. The method of claim 13, wherein depositingthe coating comprises: forming a first layer directly on the pluralityof conductive tracks, the first layer comprising a metal fluoride andhaving a thickness from 3 angstroms to 5 nanometers; and forming asecond layer comprising one or more halo-hydrocarbon polymers, thesecond layer in direct contact with the first layer, the second layerhaving a thickness from 1 nanometer to 10 micrometers.
 24. The method ofclaim 13, wherein the coating has a variable thickness such that aportion of the coating on a first region of the substrate has adifferent thickness than another portion of the coating on a secondregion of the substrate, wherein the first region of the substratecomprises a region that is distinct from the second region of thesubstrate.
 25. The method of claim 13, wherein: the plurality ofconductive tracks are copper tracks; and the at least onehalo-hydrocarbon polymer comprises a PTFE type material.
 26. The methodof claim 13, wherein the coating is deposited by plasma deposition usingone or more precursor compounds comprising at least one of: aperfluoroalkane; a perfluoroalkene; a perfluoroalkyne; a fluoroalkane; afluoroalkene; a fluoroalkyne; a fluorochloroalkane; afluorochloroalkene; and a fluorochloroalkyne.
 27. The method of claim26, wherein the one or more precursor compounds have a straight chain orbranched chain structure.
 28. The method of claim 13, wherein: thecoating is deposited as a substantially continuous layer on the at leastone surface of the substrate; the solder joint is formed at a particularregion of the substrate; and the soldering removes the coating from theparticular region of the substrate without removing the coating fromother regions of the substrate.
 29. The method of claim 13, whereinsoldering through the coating comprises heating a flux at a particularregion of the substrate, the heated flux dissolving the coating from theparticular region without removing the coating from other regions of thesubstrate.
 30. The method of claim 13, wherein soldering through thecoating comprises heating a flux at a particular region of thesubstrate, the heated flux removing the coating from the particularregion without removing the coating from other regions of the substrate.31. The method of claim 13, wherein the coating is configured towithstand multiple heat cycles.
 32. The method of claim 13, wherein thecoating is deposited by at least one of; plasma deposition; chemicalvapor deposition; metallo-organic-chemical vapor deposition; molecularbeam epitaxy; spray coating; sputtering; and spin coating.
 33. Themethod of claim 13, further comprising, after depositing the coating andprior to forming the solder joint, storing the substrate for asubstantial period of time, wherein the coating protects the pluralityof conductive tracks from oxidation during storage.
 34. The method ofclaim 13, further comprising, after forming the solder joint, storingthe substrate for a substantial period of time, wherein the coatingprotects the plurality of conductive tracks from oxidation duringstorage.
 35. The method of claim 13, further comprising wire bonding aparticular electrical component to at least one conductive track,wherein: the particular electrical component is wire bonded with atleast one wire that is coated with the coating; and the wire bond isformed without first removing the coating from the wire.
 36. A method,comprising: forming a substrate comprising an insulating material;forming a plurality of conductive tracks on at least one surface of thesubstrate to form a printed circuit board; and prior to soldering one ormore components to the plurality of conductive tracks, depositing acoating on the at least one surface of the substrate, wherein thecoating: protects the plurality of conductive tracks from oxidation; andcomprises at least one halo-hydrocarbon polymer that allows a connectionto be soldered through the coating to at least one conductive trackwithout first removing the coating from the at least one conductivetrack.
 37. The method of claim 36, wherein the coating has a thicknessfrom 1 nanometer to 10 micrometers.
 38. The method of claim 36, whereinthe coating has a thickness from 10 nanometers to 100 nanometers. 39.The method of claim 36, wherein the at least one halo-hydrocarbonpolymer comprises one or more fluoro-hydrocarbons.
 40. The method ofclaim 36, wherein less than five percent of the total number of atoms inthe at least one halo-hydrocarbon polymer are heteroatoms.
 41. Themethod of claim 36, wherein the coating comprises: a first layercomprising a metal halide, the first layer protecting the plurality ofconductive tracks from oxidation; and a second layer comprising one ormore halo-hydrocarbon polymers.
 42. The method of claim 41, wherein thefirst layer comprising a metal halide allows self fluxing duringsoldering of the at least one electrical component.
 43. The method ofclaim 36, wherein the coating has a variable thickness such that aportion of the coating on a first region of the substrate has adifferent thickness than another portion of the coating on a secondregion of the substrate, wherein the first region of the substratecomprises a region that is distinct from the second region of thesubstrate.
 44. The method of claim 36, wherein: the plurality ofconductive tracks are copper tracks; and the at least onehalo-hydrocarbon polymer comprises a PTFE type material.
 45. The methodof claim 36, wherein the at least one halo-hydrocarbon polymer is suchthat, when a flux is heated at a particular region of the substrate, thecoating dissolves from the particular region without being dissolvedfrom other regions of the substrate.
 46. The method of claim 36, whereinthe at least one halo-hydrocarbon polymer is such that, when a flux isheated at a particular region of the substrate, the coating is removedfrom the particular region without being removed from other regions ofthe substrate.
 47. The method of claim 36, wherein the electricalimpedance through the coating along an axis perpendicular to the atleast one surface of the substrate is lower than the electricalimpedance through the coating along an axis parallel to the at least onesurface of the substrate.
 48. The method of claim 36, wherein thecoating is deposited by plasma deposition.